Java source files with a date set in the future result in an uncompilable class in BlueJ. Compiling the class appears to succeed, except that the "uncompiled" stripes remain on the class and it cannot be instantiated etc.
It would be trivial to get the class to display correctly after compilation (and allow instantiation etc) but if the date is not corrected, the file will show as uncompiled again when the project is closed/re-opened. The correct fix is to either change the file date or display a warning.
(The opposite issue
- when the class file is future-dated
- is not so much a problem. BlueJ will still mark the file uncompiled if it is changed, and re-compile it, which causes the date to be reset).